In recent years there has been tremendous interest in GaN based III-N materials. A dramatic improvement in the material quality has led to the development of high brightness light emitting diodes, and more recently "blue" laser diodes. There has also been a dramatic improvement in the performance of high power microwave metal-semiconductor field-effect transistors and modulation doped field-effect transistors based on these materials.
Crucial to all these applications is the growth of material with high crystalline quality and of high purity. Various techniques have been used to grow GaN including metalorganic vapor phase epitaxy (MOVPE), plasma molecular beam epitaxy (plasma MBE), ammonia molecular beam epitaxy (ammonia-MBE), also referred to as reactive molecular beam epitaxy, and magnetron sputter epitaxy (MSE). Typically GaN epilayers are grown on sapphire substrates, which are highly lattice mismatched, necessitating the predeposition of a thin (.about.500 .ANG.) buffer/nucleation layer of either GaN or AlN. The observed electrical and optical properties of the resulting GaN layers is strongly dependent on the dislocation density and of the overall impurity content.
Using these growth techniques, room temperature electron mobilities for MOVPE-grown silicon doped GaN layers are typically reported in the range of 350-600 cm.sup.2 /V s. The highest room temperature mobility ever reported for GaN was 900 cm.sup.2 /V s deposited by MOVPE for a 4 .mu.m thick layer. In contrast, the highest room temperature mobility for plasma-MBE grown GaN is around 300 cm.sup.2 /V s and for ammonia-MBE is 350 cm.sup.2 /V s.